The investigation of physical factors affecting the success of bone marrow transplantation experiments was continued and include both radiobiological effects of the direction of the exposures and subtle changes in physical factors that alter the survival of irradited inbred mice with and without marrow graft. Basic concepts of radiation biology were demonstrated to be invalid and physical factors, considered of little consequence when applied to biological systems, were critical for the reproducibility of these experiments. Consequently the concept of the dose-rate effect can no longer be limited to fractionated and protracted irradiation where cell proliferation and repair are major factors in improved survival but must also include esposures at low dose rates over time spans too short to allow for cell proliferation and little if any repair. New parameters for "defining" x-ray treatment are required along with more precise reporting of experimental conditions as a result of these investigations.